if you already have 3rd party dns, you do not need to set ns1/ns2 all you need is www and @ A records pointing to server IP and then domain updated with your 3rd party dns providers name servers

if you want vanity @ns1/ns2 your domain name servers like at Domains - Domain name DNS setup on local NSD server | Centmin Mod Community you need to create those nameservers in your panel first so ask your dns provider about vanity nameservers as you have that wrong, creating vanity nameservers is NOT an A record, it's separately done and depends on dns control panel/provider.

with domain registrars like namecheap.com, namesilo.com and internet.bs at least when you register a domain, you also get free dns hosting, so like your dns control panel, these domain registrars provide you with their own nameservers i.e. ns1.domainregistrarprovided.com and you just add to your domain's dns settings that provided ns1 where your screenshot above had ns1 for and ip and ns1 would be provided by your domain registrar or dns manager.

If you want vanity ns1.yourdomain need to create the nameservers yourself